CHT sensor extension, Curious, does it work? Who uses one? Options

[pbanders]

Here's the schematic for using a DPDT switch and a 300 ohm pot to create a circuit that would allow you to put parallel resistance to the CHT, and lower the resistance seen by the ECU, so that the ECU thinks the car is more warmed-up than the CHT is telling it. Flipping the switch to the other position takes the pot out of the circuit and lets the CHT alone control the ECU.

EDIT: might be better to use a 500 ohm pot, they're more readily available, cheaper, and smaller. The current requirements for this application are low.



500 ohm Pot

http://www.parts-express.com/500-ohm-poten...-shaft--023-500

DPDT switch

http://www.parts-express.com/dpdt-slide-sw...-lever--023-670

Probably the best way to implement this is to run wires from the CHT and the ECU wiring harness to the passenger compartment, then use a small project box to house the pot and the DPDT switch.

Note this is STRICTLY ThEORETICAL at this moment, nobody has done this to my knowledge and it may not work. Try at your own risk/gain.

[pbanders]

I never was worth a crap as an EE, I was more of a materials guy, so circuits really aren't my bag. What I'd love to figure out is if it's possible to do a totally passive circuit that has a single pot, where when centered, there's no effect on the resistance of the CHT as seen by the ECU. Turn it to the left, and parallel resistance is added, and a lower resistance than the current CHT value is seen by the ECU. Turn it to the right, and series resistance is added, and a higher resistance than the current CHT value is seen by the ECU. Multiple pots, ganging, whatever, not sure it can be done without active switching. I have a question in at a circuits group to see if it's possible, but if a REAL EE here in this forum can figure it out, please do so (IMG:style_emoticons/default/smile.gif) .

[pbanders]

OK, here's a circuit version with two pots, one for lean adjust, one for rich. To have the CHT be in control and the pots out of the circuit, flip the DPDT switch to the other position and turn the "Rich" pot fully to the right (shunt) position.

[pbanders]

OK, I can't stop, here's one more circuit where I've added a test node and an isolation switch, so that you can test either the voltage across the CHT and series/parallel circuit, or by opening the switch, test the resistance to ground. Obviously, when the Isolate switch is opened, the car won't run because the ECU will see an open for the CHT, but you can test the resistance so that you know what the setpoint is. While the car is running, you can watch the voltage and see how it's changing as the temperature changes the resistance of the CHT or you change the adjustments on the pots and DPDT switch. Note I have the CHT at 50 ohms, that's just the approximate resistance of the CHT when the car is fully warmed-up.



SPST Switch

http://www.parts-express.com/spst-mini-pad...switch--060-532

Banana Binding Post (red, buy 2: ECU, Test Point)

http://www.parts-express.com/binding-post-red--090-485

Banana Binding Post (black, buy 2: CHT, ground)

http://www.parts-express.com/parts-express...-black--090-480

Banana Plug (red, buy 2: ECU, Test Point)

http://www.parts-express.com/parts-express...ug-red--090-465

Banana Plug (black, buy 2: CHT, ground)

http://www.parts-express.com/parts-express...-black--090-460

Project box

http://www.parts-express.com/hammond-1591a...97-083--320-702

Pot Knobs

https://smile.amazon.com/gp/product/B00NN6M...=UTF8&psc=1

[Spoke]

OK, I can't stop

I know the feeling. Here's a little spin on your updated circuit to have a rich, lean, and normal setting. Using all the same components as you did except using a center off DPDT switch.

When SW1 is in the up position, the CHT is set for lean operation. In the center (OFF) position, the CHT is set for rich operation. In the down position, the CHT is normal.

DPDT Switch ON-OFF-ON

[pbanders]

OK, I can't stop

I know the feeling. Here's a little spin on your updated circuit to have a rich, lean, and normal setting. Using all the same components as you did except using a center off DPDT switch.

When SW1 is in the up position, the CHT is set for lean operation. In the center (OFF) position, the CHT is set for rich operation. In the down position, the CHT is normal.

DPDT Switch ON-OFF-ON

Good idea, I didn't know such a switch existed! You can do the same with mine by adjusting the rich pot to be a shunt, but this is better.

[pbanders]

One thing I always know for sure: If I draw a circuit, somebody else can draw it better (IMG:style_emoticons/default/smile.gif) . That's why I ran away from design as fast as I could.

[TX914]

Very nice. This seems closer to being realized. I gather Spoke’s revised UI switch still allows for adjusting lean and rich circuits.

I’m curious about the Isolate switch as you say the car would not run when open. What would be the effect of grounding the ECU CHT sensor lead on hot restart?
I recall long ago when experiencing some intermittent bucking on acceleration, to test for a bad CHT sensor (engine hot) my mechanic simply disconnected the ECU lead and grounded it to neg. battery terminal
(or was it just to ground – can’t remember), and the car ran fine.

[timothy_nd28]

Instead of a switch, why not a small circuit that selects rich or lean based on engine temperature? It would keep everything in the engine bay

[pbanders]

Very nice. This seems closer to being realized. I gather Spoke’s revised UI switch still allows for adjusting lean and rich circuits.

I’m curious about the Isolate switch as you say the car would not run when open. What would be the effect of grounding the ECU CHT sensor lead on hot restart?
I recall long ago when experiencing some intermittent bucking on acceleration, to test for a bad CHT sensor (engine hot) my mechanic simply disconnected the ECU lead and grounded it to neg. battery terminal
(or was it just to ground – can’t remember), and the car ran fine.

Surprisingly, you could actually do that, because the CTC circuit in the ECU's response to the value of the CHT resistance flattens out below 300 ohms, so anything under 300 ohms has the same effect, including a short. As the CHT gets down to 50 ohms or so when the engine is fully warmed up, a short would act the same.

You can see all of this in the analysis of the CTC circuit in the ECU on my web page.

http://members.rennlist.com/pbanders/ecu.htm#CTC

This behavior is by design so that once the engine is warmed up, the mixture doesn't go all over the place as the head temperatures rise and fall, which anyone with a CHT probe will tell you is commonplace. What we think is happening (as of yet unproven) with the hot-start problem is that the CHT is cooling off enough that it's getting to more than 300 ohms, and leaning out the mixture. By having the test point in the circuit and the isolate switch, I'll be able to see if this is really what's happening, or like with many things we've thought we understood with these cars, we had no idea of what we were talking about (IMG:style_emoticons/default/smile.gif) .

Remember, experimental results always trump theoretical musings.

[pbanders]

Instead of a switch, why not a small circuit that selects rich or lean based on engine temperature? It would keep everything in the engine bay

Post a schematic and we'll discuss it (IMG:style_emoticons/default/smile.gif)

[timothy_nd28]

[TX914]

I'll take 2 of those. (IMG:style_emoticons/default/smile.gif)

[pbanders]

Tim, thanks, I've got all those components except the Scarab beetles. I ordered those on Amazon Prime, they're coming by drone later today, I'll let you know how it works.

[timothy_nd28]

Sorry, I know it wasn't helpful but I happen to love that schematic (IMG:style_emoticons/default/biggrin.gif)

I have a better idea. Consider a circuit that replaces your manual switch. This circuit most likely micro controller, would listen to the throttle position sensor. Before starting the car, you could depress the accelerator pedal 3 times, which would be recognized by the controller for lean behavior, 4 time for rich. The controller could be programmed to default back to normal conditions after a default time.

[nine9three]

I have the extension and it transformed my car when cold from a hunting idle to smooth as glass. No other issues when warm and still stable. All other components of the injection system function properly and are within spec. Because the ohm's change quickly with temperature there seems to be no issue with the described quick restart after warm rest. My experience has better than expected.

[Mikey914]

I have the extension and it transformed my car when cold from a hunting idle to smooth as glass. No other issues when warm and still stable. All other components of the injection system function properly and are within spec. Because the ohm's change quickly with temperature there seems to be no issue with the described quick restart after warm rest. My experience has better than expected.

Curious, I wonder if the real issue was a poor connection. The ohms drop off quickly above 150. For the head to hit 150, the engine is pretty warm as this is an external temp. If the issue was cold start and the difference is night and day maybe the wire would be suspect? This should only delay the curve, not improve it.

[913B]

I have the extension and it transformed my car when cold from a hunting idle to smooth as glass. No other issues when warm and still stable. All other components of the injection system function properly and are within spec. Because the ohm's change quickly with temperature there seems to be no issue with the described quick restart after warm rest. My experience has better than expected.

+1 I totally agree after installing a new CHT and extender, idle was still not so perfect until I cut that dang plastic connector where the CHT plugs into and but connected together, the idle is very steady with no fluctuations for a 40 yr old FI, couldn't be happier, ok maybe little more HP (IMG:style_emoticons/default/biggrin.gif)

[Mikey914]

What I'm getting at is you installed a new CHT, or perhaps allowed the wire it make contact correctly. Sure there will be a delay on the curve, and that may make it more drivable, but the magic bullet was not the extender.

[pbanders]

I have the tester box all put together, not wired yet. Didn't do Spoke's improvement because I couldn't get the center off switch added to my order, so it's not that elegant, but it still will work. Once I get it done, I'll start doing some testing, I should be able to have some experimental evidence to validate or refute a number of driveability issues we've attributed to rich and lean conditions.

Oh, and as always, I wish I'd used a slightly bigger project box, because it ended up being a tight fit, and I didn't have a lot of options on where to put some of the components, but it's ok as is.